The present invention relates generally to communication systems, and more particularly to wireless communication systems such as code division multiple access (CDMA) systems for fixed wireless loop (FWL) and other applications.
FIG. 1 shows a portion of a conventional omni-beam FWL system 10. The portion of system 10 shown includes four hexagonal cells 12-1, 12-2, 12-3 and 12-4, each with a corresponding base station 14-1, 14-2, 14-3 and 14-4, and a subscriber unit 16. The system 10 will generally include numerous additional cells, base stations and subscriber units configured in a similar manner. It is assumed in this system that the base stations are equipped with omni-directional antennas, and that the positions of the subscriber units are fixed. The base station 14-3 of FIG. 1 is in communication with the subscriber unit 16 in cell 12-3, e.g., for providing a communication channel for an on-going voice or data call. The omni-beam FWL system 10 may be configured using a number of different techniques.
FIG. 2 shows an example of how the omni-beam FWL system 10 may be implemented using a time division multiple access (TDMA) technique such as that used in the Digital European Cordless Telephone (DECT) standard. In accordance with this TDMA technique, different frequencies are used for the different cells, such that among the cells, users are separated in frequency. A suitable frequency reuse pattern, e.g., a seven-cell hexagonal reuse pattern, may also be used in order to limit the number of different frequencies required. Within a given cell, users are separated in time through the use of a sequence of time slots 20, including time slots 22-1, 22-2, . . . 22-N. The system 10 may also be implemented using a code division multiple access (CDMA) technique. In accordance with this technique, the same frequencies but different codes are used for each of the cells, such that the codes are used to separate users in different cells and within a given cell. Some frequency separation may also be used in conjunction with the code separation in order to reduce interference from other cells. Additional details regarding conventional CDMA systems are described in, for example, Andrew J. Viterbi, xe2x80x9cCDMA: Principles of Spread Spectrum Communication,xe2x80x9d Addison-Wesley, 1995, which is incorporated by reference herein. Other conventional CDMA systems are described in, for example, TIA/EIA/IS-95A, xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d June 1996, and ANSI J-STD-008, xe2x80x9cPersonal Stationxe2x80x94Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems,xe2x80x9d both of which are incorporated by reference herein.
FIG.3 shows a conventional narrow-beam FWL system 30. The portion of system 30 shown includes four hexagonal cells 32-1, 32-2, 32-3 and 32-4, each with a corresponding base station 34-1, 34-2, 34-3 and 34-4. In this system, it is again assumed that the positions of the subscriber units are fixed. The base stations in system 30 are equipped with directional antennas which generate narrow beams 36. At any given time, only a subset of the total number of beams in the system is active, i.e., communicating with users. The beams 36 are made as narrow as possible in order to target only a single user, and thereby minimizing inter-cell interference. In order to provide an increased capacity, the system 30 may be configured such that all cells use the same frequencies, i.e., a frequency reuse factor of 1. FIG. 4 shows an alternative implementation in which a given cell 42-i includes nine electronically-steerable narrow beams 46. The beams 46 are separated into three sectors, each including three beams designated 1,2 and 3. This provides a more manageable hopping pattern, e.g., turning on a designated single beam within each sector at any given time.
FIGS. 5 and 6 illustrate the difference between sectorization and steerable beams in a narrow-beam system such as system 30 of FIG. 3, which assumes a frequency reuse factor of 1. FIG. 5 shows a pair of sectorized cells 50-1 and 50-2 having base stations 52-1 and 52-2, respectively. In this example, a beam 53 from one of six sectors of the cell 50-1 and a beam 55 from one of the six sectors of the cell 50-2 will generate co-channel, i.e., inter-cell, interference. If the beams are sectorized but not steerable, then it is generally not possible to mitigate this type of co-channel interference adaptively unless the sectors are separated in frequency. FIG. 6 shows an arrangement in which a pair of cells 60-1 and 60-2, via respective base stations 62-1 and 62-2, generate sectorized and steerable beams. It can be seen that, as illustrated by the relative positions of steerable beams 63 and 65, that such an arrangement can be used to provide adaptive mitigation of co-channel interference.
FIG. 7 illustrates a conventional technique for separating uplink (UL) and downlink (DL) traffic for a given antenna beam in an omni-beam or narrow-beam system. In this technique, an uplink channel 72U and a downlink channel 72D are separated in frequency as shown, i.e., frequency division duplexing (FDD) is used to separate uplink and downlink traffic. Users of the uplink and downlink channels 72U and 72D are separated in time, using sequences of time slots 74-1, 74-2, 74-3 . . . and 76-1, 76-2, 76-3 . . . , respectively.
The conventional techniques described above suffer from a number of disadvantages. For example, it is generally very difficult to generate narrow beams targeted to single users, as in the narrow-beam FWL system 30 of FIG. 3. In addition, narrow beams of this type are susceptible to increased interference from effects such as shadowing and problematic sidelobes. Use of narrow beams in conjunction with a TDMA technique within a given cell can lead to catastrophic interference. For example, if beams from adjacent cells overlap, there is catastrophic interference since the signals are neither separated in frequency nor in time among the different cells, but are instead separated in the spatial domain. In a high density environment, this limitation can severely restrict capacity. Another problem is that conventional FDD techniques, such as those used to separate uplink and downlink in FIG. 7, generally cannot adaptively tradeoff capacity between uplink and downlink. As a result, these FDD techniques are generally not well suited for use with, e.g., data-oriented wireless services. It is apparent from the foregoing that further improvements are needed in wireless communication techniques in order to overcome these and other problems of the prior art.
The invention provides apparatus and methods for wireless communication in fixed wireless loop (FWL) and other types of systems in which, e.g., information is communicated in a given cell of the system between subscriber units and a base station over an uplink and a downlink. In accordance with a first aspect of the invention, a code division duplex (CDD) time-slotted CDMA wireless communication system is provided. Communications on the uplink are separated from communications on the downlink using code division duplexing, and communications with different subscriber units in the cell are separated using a code division multiple access technique, e.g., time-slotted CDMA. The code division duplexing may be implemented by, e.g., assigning a first subset of a set of codes to the uplink and a second subset of the set of codes to the downlink. The code assignment process may be repeated for different time slots, such that the number of codes in the first and second subsets varies across the time slots in accordance with uplink and downlink traffic demands. The system may utilize electronically-steered beams generated by antennas associated with the base stations. Any particular beam at a given time may have a width sufficient to provide simultaneous coverage for at least n of the subscriber units at that time, where n is greater than or equal to two. The n subscriber units are assigned different codes as part of the code division multiple access technique.
In accordance with another aspect of the invention, a time division duplex (TDD) time-slotted CDMA wireless communication system is provided. Communications on the uplink are separated from communications on the downlink using time division duplexing, and communications with different subscriber units in the cell are separated using a code division multiple access technique, e.g., time-slotted CDMA. The time division duplexing may be implemented by, e.g., assigning a first subset of a set of time slots to the uplink and a second subset of the set of time slots to the downlink. The time slot assignment process may be implemented such that the assignment of time slots to uplink and downlink is varied in accordance with uplink and downlink traffic demands. A TDD time-slotted CDMA system in accordance with the invention may also make use of the above-noted electronically-steered beams, each having a width sufficient to provide simultaneous coverage for at least n subscriber units at a given time.
In accordance with another aspect of the invention, an orthogonal frequency division multiplexing (OFDM) wireless communication system is provided. Communications on the uplink are separated from communications on the downlink using OFDM. Subscriber units in the cell are separated using, e.g., code division multiple access, time division multiple access, frequency division multiple access or combinations of these and other techniques. The OFDM may involve, e.g., assigning a first subset of M OFDM carriers to the uplink and a second subset of the M carriers to the downlink. The carrier assignment process may be repeated for different time slots, such that the number of carriers in the first and second subsets varies across the time slots in accordance with uplink and downlink traffic demands.
The invention provides improved performance in wireless communication systems, particularly in applications involving heterogeneous traffic, e.g., mixed voice and data traffic, and other applications in which uplink and downlink capacity requirements are subject to large fluctuations. The invention is particularly well suited for use in applications such as omni-beam and narrow-beam FWL systems, although it can provide similar advantages in numerous other wireless communication applications.